In the aforementioned patents, a method is disclosed for producing composite metal powders comprised of a plurality of constituents mechanically alloyed together such that each of the particles is characterized metallographically by an internal structure in which the starting constituents are mutually interdispersed within each particle. In general, production of such composite particles involves the dry, intensive, high energy milling of powder particles such that the constituents are welded and fractured continuously and repetitively until, in time, the intercomponent spacing of the constituents within the particles can be made very small. When the particles are heated to a diffusion temperature, interdiffusion of the diffusible constituents is effected quite rapidly.
The potential for the use of mechanically alloyed powder is considerable. It affords the possibility of improved properties for known materials and the possibility of alloying materials not possible, for example, by conventional melt techniques. Mechanical alloying has been applied to a wide variety of systems containing, e.g., elemental metals, non-metals, intermetallics, compounds, mixed oxides and combinations thereof. The technique has been used, for example, to enable the production of metal systems in which insoluble non-metallics such as refractory oxides, carbides, nitrides, silicides, and the like can be uniformly dispersed throughout the metal particle. In addition, it is possible to interdisperse within the particle larger amounts of alloying ingredients, such as chromium, aluminum and titanium, which have a propensity to oxidize easily. This permits production of mechanically alloyed powder particles containing any of the metals normally difficult to alloy with another metal. Still further marked improvements in the mechanically alloyed materials can be obtained by various thermomechanical treatments which have been disclosed. U.S. Pat. Nos. 3,814,635 and 3,746,581, for example, involve methods of processing the powders to obtain stable elongated grain structures.
Notwithstanding the significant achievements in properties that have been obtained by the mechanical alloying technique, research efforts continue in order to improve the mechanical alloying technique and the properties of the alloys made by this technique and to improve the economic feasibility of producing the alloys commercially.
One aspect of this invention involves the processing level of the mechanically alloyed powders, another the window permissible for thermomechanical treatment of such powders. By window, is meant the range of thermomechanical treatment parameters which can be applied to produce material meeting target properties.
As indicated above, a characteristic feature of mechanically alloyed power is the mutual interdispersion of the initial constituents within each particle. In a mechanically alloyed powder, each particle has substantially the same composition as the nominal composition of the alloy. The powder processing level is the extent to which the individual constituents are commingled into composite particles and the extent to which the individual constituents are refined in size. The mechanically alloyed powder can be overprocessed as well as underprocessed. An acceptable processing level is the extent of mechanical alloying required in the powder such that the resultant product meets microstructural, mechanical and physical property requirements of the specific application of the alloy. Underprocessed powders, as defined herein, means that the powder is not readily amenable to a thermomechanically process treatment which will form a clean desirable microstructure and optimum properties. Overprocessed powder is chemically homogeneous, the deformation appearance is uniform, and it can under certain conditions be processed to a clean elongated microstructure. However, the conditions under which the material can be processed to suitable properties--i.e. the thermomechanical processing window--is narrower. It will be obvious to those skilled in the art that for commercial processing of alloys standardized conditions are required for thermomechanical processing. Therefore, the size of the window for processing to target properties is very important. Furthermore, since the properties of the material are determined only after consolidation and thermomechanical processing, both the processing level in the powder and the window for thermomechanical processing are very important elements in making the production of mechanically alloyed materials commercially feasible from an economic standpoint.
Typical measures of processing level are powder hardness and powder microstructure. Saturation hardness is the asymptotic hardness level achieved in the mechanically alloyed powder after extended processing. Saturation hardness is actually a hardness range rather than an absolute value. In other words, it is a hardness regime that no longer shows a sharp increase with additional processing. Overprocessed powder is well into the saturation hardness region. It is not necessary to reach saturation hardness level in order to achieve mechanically alloying. The significance of saturation hardness resides in its relationship to the setting up of standardized conditions to thermomechanically treat compacted powders in order to achieve target properties, e.g. of strength and/or microstructure.
With respect to microstructure of the powder, the powder can be processed to a level where, for example, at a magnification of 100.times., it is substantially homogeneous chemically, or further until it is "featureless". Featureless, mechanically alloyed powder has been processed sufficiently so that substantially all the particles have essentially no clearly resolvable details optically when metallographically prepared, e.g. differentially etched, and viewed at a magnification of 100.times.. That is, in featureless particles distinctions cannot be made in the chemistry, the amounts of deformation, or the history of the constituents. As in the case of saturation hardness, the term featureless is not absolute. There are degrees of "featurelessness" and a range within which a powder can be considered optically featureless at a given magnification.
Dry, intensive, high energy milling required to produce mechanical alloying is not restricted to any type of apparatus. Heretofore, however, the principal method of producing mechanically alloyed powders has been in attritors. An attritor is a high energy ball mill in which the charge media are agitated by an impeller located in the media. In the attritor the ball motion is imparted by action of the impeller. Other types of mills in which high intensity milling can be carried out are "gravity-dependent" type ball mills, which are rotating mills in which the axis of rotation of the shell of the apparatus is coincidental with a central axis. The axis of a gravity-dependent type ball mill (GTBM) is typically horizontal but the mill may be inclined even to where the axis approaches a vertical level. The mill shape is typically circular, but it can be other shapes, for example, conical. Ball motion is imparted by a combination of mill shell rotation and gravity. Typically the GTBM's contain lifters, which on rotation of the shell inhibit sliding of the balls along the mill wall. In the GTBM, ball-powder interaction is dependent on the drop height of the balls.
Early experiments appeared to indicate that, while mechanical alloying could be achieved in a GTBM, such mills were not as satisfactory for producing the mechanically alloyed powder as attritors in that it took a considerably longer time to achieve the same processing level.
Comparative merits of processing powders in a GTBM were based on experience with attrited powders. While mechanical alloying can be achieved without processing to saturation hardness, in work on consolidated attrited powder it was found that the powder had to be processed to essentially saturation hardness. It was also found that the attrited powder had to be processed to a substantially featureless microstructure as defined herein; i.e., when viewed metallographically at 100.times. magnification. A failure to carry out the processing in the attritor to this degree increases the chances of producing an ultimate consolidated product which does not meet the target properties. For example, it might be difficult to produce a clean microstructure from underprocessed attrited powder. However, as indicated above, like saturation hardness, the "featureless" appearance of the powders is not an absolute characteristic--rather, it is a range. And, the exact degree into the "featureless" range which must be achieved in order to have an acceptable processing level is not easily determined. On the other hand, it is possible to overprocess the powders and overprocessed powder narrows the window for thermomechanical processing to target properties. With attrited powder--although possible--it has been difficult to standardize thermomechanical processing conditions on a commercial scale for a given alloy, and the determination of whether the acceptable processing level has been achieved for each batch of alloy can only be determined easily after the final step in the processing.
It has now been found that when the processing conditions are properly chosen, the GTBM can be a preferred route to achieve mechanical alloying to an acceptable processing level. It has also been found that when processing powder in a GTBM, it is not necessary to process powder to the same processing level as in the attritor in order for the powder to achieve an acceptable processing level. Also, powder mechanically alloyed in a GTBM reaches an acceptable processing level at lower levels of hardness than necessary in an attritor. Moreover, since the window for thermomechanical treatment is larger, the powders mechanically alloyed in a GTBM lend themselves to more predictable properties for a given such treatment and to greater flexibility in conditions for thermomechanical treatment. Thus, for many purposes, it is more feasible economically to produce commercial quantities of mechanically alloyed powders in a GTBM than in an attritor.
Another advantage resulting from the lower acceptable processing level is that at the acceptable point the level of processing can be more clearly defined for powders produced in a GTBM because the powder exhibits features when viewed microstructurally. Thus, it is easier to discriminate between the different processing levels
It is believed that one reason for the improved processing level factor of the GTBM-produced powder may be that the processing level distribution of the powder particles is narrower than for attritor-produced powders.
Although, as described below, the process of the present invention is applicable to the production of a wide variety of mechanically alloyed powder compositions of simple and complex alloy systems, it will be described with reference to nickel-, iron- and copper-base alloy systems, and with particular reference to nickel-base oxide dispersion strengthened superalloys.